The present invention relates in general to magnetic field and current sensors and more particularly to magnetic field sensors that utilize chip or conductive straps to apply a current induced field to the sensor.
Magnetic field sensors have applications in magnetic compassing, ferrous metal detection, and current sensing. They may detect magnetic field variations in machine components, the earth""s magnetic fields, underground minerals, or electrical devices and lines.
In these situations, one may use a magnetoresistive (xe2x80x9cMRxe2x80x9d) sensor that is able to detect small shifts in magnetic fields. Such MR sensors may be formed using typical integrated circuit fabrication techniques. Typically, MR sensors use permalloy, a ferromagnetic alloy containing nickel and iron, as the magnetoresistive material. Often, the permalloy is arranged in thin strips of permalloy film. Such strips are usually several times longer than they are wide, and each strip will have a longxe2x80x94or xe2x80x9ceasyxe2x80x9dxe2x80x94axis and a short axis. Moreover, through the design of the strips or through the use of an external field, the strips may often be magnetized in a particular directionxe2x80x94for example the easy axisxe2x80x94while in a default or reset state. Even so, an external magnetic field may act on a strip to rotate its magnetization direction away from the reset state magnetization direction.
When a current is run through an individual strip, the magnetization direction of the strip may form an angle with the direction of current flow. As the magnetization direction changes, the effective resistance of the strip changes. Particularly, a magnetization direction parallel to the current flow direction results in maximum resistance through the strip and a magnetization direction perpendicular to the current flow direction results in minimum resistance through the strip.
Therefore, when an external magnetic field acts on a permalloy strip to rotate its magnetization direction, the resistance of the strip may change. This changed resistance may cause a change in voltage drop across the strip when a current is run through the strip. This change in voltage may be measured as an indication of change in the magnetization direction of external magnetic fields acting on the strip.
To form the magnetic field sensing structure of a MR sensor, several permalloy strips may be electrically connected together. The permalloy strips may be placed on the substrate of the MR sensor as a continuous resistor in a xe2x80x9cherringbonexe2x80x9d pattern or as a linear strip of magnetoresistive material, with conductors across the strip at an angle of 45 degrees to the long axis of the strip. This latter configuration is known as xe2x80x9cbarber-pole biasing.xe2x80x9d It may force the current in a strip to flow at a 45-degree angle to the long axis of the strip, because of the configuration of the conductors. These sensing structure designs are discussed in U.S. Pat. No. 4,847,584, Jul. 11, 1989, to Bharat B. Pant and assigned to the same assignee as the current application. U.S. Pat. No. 4,847,584 is hereby fully incorporated by reference.
Whatever the configuration of the magnetoresistive material, the magnetization direction of the materials must be set in a single domain state for the purposes of repeatability of measurement. Unfortunately, this domain setting may be upset after manufacture by the presence of powerful magnetic fields near the magnetoresistive material. Though, in certain designs, large external magnets can be specifically placed to reset the domain setting, this may not be feasible when the MR sensor has already been packaged into a system. Particularly, some situations require several sensors within a single package to be magnetized in opposite directions. In this case, one can wrap individual coils around each sensor, but individual coils are expensive and are often unable to generate the large fields required to reset the sensors.
Alternately, current straps, known as set-reset straps and offset straps, may be used for resetting the domain state of an MR sensor and for biasing an MR sensor, respectively. U.S. Pat. No. 5,247,278 also to Bharat B. Pant, discloses the use of current straps for this purpose. U.S. Pat. No. 5,247,278 is hereby fully incorporated by reference.
Set-reset straps and offset straps may be utilized before every measurement, if desired. However, in some applications, it may be preferable to use set-reset and/or offset straps only in certain instances, such as upon power-up of an MR sensor device. Similarly, the occurrence of an anomalous magnetic field may be an appropriate time for using set-reset straps and/or offset straps, in order to reset the domain state of an MR sensor and/or to bias an MR sensor. The set-reset and offset straps could also be used for compensating for stray fields, calibration, current measurements, or magnetic initialization (startup).
These set-reset straps and offset straps provide a more efficient means of resetting and biasing an MR sensor than external means that are larger and more expensive. Moreover, because the straps may be formed by deposition on the same substrate as the MR sensor, the straps may be located closer to the magnetic field sensing structure than other means might be. This may allow for less energy to be used in resetting or biasing the MR sensor.
Further, because of size or other constraints, these straps may be only a few microns thick. Unfortunately, because of a need for a certain strength magnetic field to reset or bias a magnetic field sensing structure, the straps may not be of sufficient size to maintain the amount of current flow necessary to generate the required reset or biasing magnetic field. Moreover, even if straps are able to maintain this current flow, the design of the device may not allow for a power source with the ability to generate such a current or to generate such a current for a desired amount of time.
In one embodiment of the present invention, a method for manufacturing a magnetic field sensor includes the step of providing a keeper material proximate to a magnetic field sensing structure. The sensor includes a substrate, a current strap, and the magnetic field sensing structure.
Another embodiment of the present invention provides either a set-reset strap or an offset strap as the current strap. This embodiment may also include both a set-reset strap and an offset strap in the same sensor.
In a preferred embodiment of the present invention, the magnetic field sensing structure also includes permalloy strips electrically connected to one another and to an output terminal, where a magnetic field indication is produced.
The foregoing and other features and advantages of the system and method will be apparent from the following more particular description of preferred embodiments of the system and method as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.